Yes, I've heard good things about pestene powder too, and also sulphur mixed in with baby oil or animal fat, rubbed through the hair. I'm not sure if "cattle coat" or "pony coat" does lice, but we use it up here for Buffalo Fly, its a mix of essential oils, and is quite potent. If its only a few cows, you could look at any of the proprietary mixes at the Chemist, and bath the cow(s). Where have they gotten the lice from? Birds? If you knock this infestation on the head, can you stop the reinfection from the source?
Let us know how you get on.